Former U.S. Secretary of State Hillary Clinton (R) and Lee Saunders (L), president of the American Federation of State, County and Municipal Employees speak during an event at the Center for American Progress March 23, 2015 in Washington, DC. (Photo by Win McNamee/Getty Images)

If you’re looking for signs that Hillary Clinton might about have sewn up the Democratic nomination for president, keep your eyes on organized labor.

Bernie Sanders may have fashioned himself as the champion of the working class, but it’s Clinton who is winning most of the endorsements from unions.

“That Bernie isn’t racking up the endorsements right now likely stems from a combination of genuine support for Clinton, plus some cold, clear-eyed calculations that she’s the likely nominee,” says Jake Rosenfeld, a sociologist at Washington University in St. Louis who has studied unions.

Last Friday, Clinton won the backing of the American Federation of State, County and Municipal Employees (AFSCME), the nation’s largest public employees union. That added to her haul of endorsements that already included the National Education Association, the American Federation of Teachers and unions representing carpenters and machinists, among others.

It’s significant that AFSCME waited until Joe Biden made his non-candidacy official, says Art Sanders, a political scientist at Drake University in Des Moines.

“A lot of the unions have come to the conclusion that Sanders is not going to be the nominee and Clinton likely is,” Professor Sanders says. “They don’t want to have to court her after endorsing Sanders, and Sanders losing.”

Unions are a declining force in American life. Their share of the labor force is down to 11 percent, compared with 20 percent back in 1983, according to the Bureau of Labor Statistics. Unions are also under attack politically, with Michigan and Wisconsin enacting right-to-work laws over the past couple of years that undercut labor’s ability to organize. Half the states now have right-to-work laws on the books.

Wisconsin Gov. Scott Walker’s successful attack on the ability of most public sector unions to bargain on behalf of workers formed the rationale for his abortive GOP presidential bid. The Supreme Court will hear a case this term that may deal a death blow to the ability of public sector unions to collect dues.

For all these reasons, unions are highly motivated to have an ally in the nation’s highest office. Their hearts might be with Sanders, but their heads are betting on Clinton.

“Many union leaders believe that Hillary Clinton is the Democrat most capable of winning in November, so they want to make sure she wins the nomination,” says Anthony Nownes, an expert on political endorsements at the University of Tennessee. “Most unions simply do not believe that Bernie Sanders can win the nomination and they are behaving accordingly.”

Organized labor has a long history of working with Clinton (as well as her husband), both when she was a senator from New York and when she ran for president the first time. Clinton received more union support in 2008 than Barack Obama. Unions may not have been able to swing that battle Clinton’s way last time, but their support still matters. There are still millions of union members who can serve as a major source of on-the-ground support for campaigns.

“Unions are in a weakened condition, but they remain one of the absolute pillars of voter mobilization for Democrats in some states,” says Nelson Lichtenstein, a labor historian and director of the Center for the Study of Work, Labor and Democracy at UC Santa Barbara. “In Nevada, for example, they are absolutely central to that state being purple and turning blue.”

That’s not to say every single worker will fall in line behind his or her union’s formal endorsement. A few thousand teachers signed an online petition protesting AFT’s early embrace of Clinton. Last month, the International Association of Fire Fighters informed the Clinton campaign that it was holding off on an endorsement, due to dissension in the ranks.

“There’s a terrific rank-and-file movement in almost all these unions for Bernie,” Lichtenstein says.

Lots of union households will end up supporting the Republican nominee. But the action for organized labor right now is on the Democratic side.

On Monday, Sanders joined a rally of union workers picketing Verizon in New York. Last week, he picked up support from a couple of unions locals in the early primary state of New Hampshire.

“When you stand with the middle class and working families and are prepared to take on powerful special interests, people will come out to vote in large numbers,” Sanders said Saturday at the Jefferson-Jackson dinner in Des Moines.

Sanders has said that Obama’s “biggest mistake” was not keeping active the legions of organizers and supporters he drew to his 2008 campaign and has vowed not to make the same mistake. “No president alone can effectively address the crises facing the working families of this country without a powerful grassroots movement,” he tweeted this month.

But the largest organized force among Democrats is backing Clinton. She has moved left on issues important to labor such as her recent call to kill the so-called Cadillac tax on generous health insurance policies and her newfound opposition to the Trans-Pacific Partnership trade deal. She’s also a proponent of minimum wage increases and mandated paid sick leave.

“Sanders may be the candidate of the working class, but they’re really not that far apart on domestic policy issues,” says Art Sanders, the Drake professor.

Clinton, in other words, is wholly acceptable to most officials in the labor movement. They’re going with that feeling, just as they did back in 2000, when most unions gave their backing to Al Gore over Bill Bradley, another early season hero of progressives.

“It’s just the judgment of various union leader types that Hillary’s going to make it and Bernie won’t,” Lichtenstein says. “They’ve been doing it for generations.”